Spinal fusion, also known as spondylodesis or spondylosyndesis, is a surgical technique used to combine two or more vertebrae. Spinal fusion is done most commonly in the lumbar region of the spine, but it is also used to treat cervical and thoracic problems. There are two main surgical procedures of spinal fusion, which may be used in conjunction with each other:                Posterolateral fusion places a bone graft in between the vertebrae through a posterior or lateral approach. These vertebrae are then fixed in place with screws and/or wire through the pedicles of each vertebra attaching to a metal rod on each side of the vertebrae.        Interbody fusion places a bone graft between the vertebrae in the area usually occupied by the intervertebral disc. In preparation for the spinal fusion, the disc is removed entirely. A cage may be placed between the vertebrae to maintain spine alignment and disc height. The intervertebral cage may be made from either a polymeric material or titanium. The fusion then occurs between the endplates of the vertebrae.        Using both types of fusion is known as 360-degree fusion. Fusion rates are higher with interbody fusion.        
The fusion process typically takes 6-12 months after surgery. During this time external bracing (orthotics) may be required.
Spinal fusion systems are known in the state of the art and are routinely used by spine surgeons to keep adjacent vertebrae in a desired position while spinal fusion takes place.
Spinal fusion systems can consist of a spinal fusion cage, which is placed between two adjacent vertebrae to facilitate spinal fusion. Spinal fusion systems can also consist of a rod or a plate that is connected to two adjacent vertebrae, to obtain fixation of the vertebrae with respect to each other, and can consist of a combination of both a spinal fusion cage and a rod or a plate. This invention is directed to spinal fusion systems comprising a spinal fusion cage alone or in combination with a rod or a plate.
Most common spinal fusion systems are made from metals, such as titanium or cobalt chrome alloys, or from a polymer that is commonly used in biomedical implants; polyetheretherketone (PEEK). These implant materials have a modulus which is much higher than that of bone and there is clinical evidence of implant subsidence and movement which is believed to be attributable to mechanical incompatibility between natural bone and the implant material. Also bone pressure necrosis does occur as a result of the presence of these metal implants.
Implants based on bone material from a donor (allograft) or from the patient itself (autograft) do have an inconsistent mechanical strength and show subsidence over time. The inconsistent properties of these implants make them generally unpredictable, challenging to reliably machine and especially prone to migration and explusion due to the difficulty of consistently machining teeth into the upper and lower implant contact surfaces.
There are disclosures of porous ceramic materials, for instance in US2005/0049706, that try to provide a spinal fusion system with mechanical properties comparable to that of bone. These implants are, however, still produced of very rigid and stiff material. These spinal fusion systems are not compressible and will still cause subsidence into the bone.
Spinal fusion cages made of porous polymeric material or a polymeric material with holes in it are described in respectively
US 2005/0246021 and EP1818024. These cages are made of bioresorbable polymeric materials, like PLA, which materials have the disadvantage that the mechanical properties of the materials are not stable because of the resorption in the body. The polymeric material itself is not compressible. The purpose of using of a porous polymeric material is that the material can be compressed in a controlled manner to create various structural patterns within the material. The porous cages and the cages with the holes will be reduced in height permanently under a load. These spinal fusion cages are thus not designed to endure a load and will show (further) permanent deformation under loading conditions.
Therefore, there is still a need for a biostable spinal fusion cage which has a tensile modulus comparable to that of bone, which does not subside and provides a good stability.
These spinal fusion cages are provided for according to the present invention.